The broad objective of this proposal is to increase our understanding of bacterial polysaccharide synthesis in pathogenic microorganisms. The K1 antigen, or polysialic acid capsule is a homopolymer of alpha2,8- linked sialyl residues. Synthesis of the capsule depends on about 15 genes of the kps pathogenicity locus that is organized into at least two convergently transcribed operons, one of which is controlled by the thermoregulated kpsF promoter. The anti-recognition functions of sialic acids (anti-phagocytosis; inhibition of antibody-independent complement fixation; molecular mimicry of host antigens) make these unique nine- carbon carboxy sugar acids important determinants of virulence in diverse human and animal pathogens. Escherichia coli K1 is a leading cause of neonatal meningitis and a frequent cause of urinary tract infections in children and bacteremia in adults. Vaccination against these diseases may not always be practical, and conventional drug therapy is problematic. Alternative or improved therapies may emerge from a better understanding of capsular polysaccharide expression. Objectives of the application are to understand the molecular mechanisms of capsule thermoregulation and sialic acid precursor biosynthesis. This information is necessary for construction of defined mutants that will allow us to unambiguously determine the association between capsule and lipopolysaccharide O antigen during disease. To accomplish these objectives, we will pursue three specific aims. First, thermoregulation will be investigated with an in vitro transcription system designed to determine the intrinsic sensitivity of the kpsF promoter to temperature. The role of a positive regulator will be determined by isolation and characterization of mutants that no longer regulate the kpsF promoter. Second, the origin of sialic acid biosynthetic precursor, N- acetylmannosamine, will be determined by genetic and biochemical approaches. Finally, the role of the capsule and its association with O antigen will be determined using defined mutants in an animal model of E. coli K1 meningitis and bacteremia. At the completion of this research, we expect to have clearly delineated the relative contributions of cell surface polysaccharides to pathogenesis. We also expect to have identified specific new targets for potential therapeutic developments aimed at blocking capsule expression in vivo.